CCM composite

ABSTRACT

CCM formed of a membrane having on one of the two surfaces a first electrocatalytic layer containing a catalyst and on the other one a second electrocatalytic layer containing a catalyst, said electrocatalytic layers and said membrane containing (per)fluorinated ionomers, said CCM having the following characteristics: size variations, for both the orthogonal directions of the plane xy, lower than 15%, by dipping the CCM, after drying at 105° C. under vacuum for one hour in demineralized water at 100° C. for 30 minutes; the CCM remains substantially unchanged after having been subjected to treatments of 60 cycles by dipping in water at 80° C. for 8 hours and then in water at the temperature of 25° C. for 16 hours.

The present invention relates to a CCM composite (Catalyst CoatedMembrane), to be used in electrochemical cells, comprising a membranehaving the two surfaces coated, partially or totally, with a catalyticor electrocatalytic layer containing a catalyst. The CCMelectrocatalytic layers of the present invention are placed onto themembrane so that the perimeter orthogonal projection of the firstelectrocatalytic layer on the second one is separated from the perimeterof the latter of at least 1 mm, preferably 2 mm in each point of saidprojection. More specifically, the CCM of the present invention shows animproved adhesion among the layers and very limited size variations inthe two orthogonal directions of the plane xy, lower than 15% andpreferably lower than 10%. Further, the CCM of the present inventionsubjected to treatments of 60 cycles by dipping in water at 80° C. for 8hours and then in water at a temperature of 25° C. for 16 hours, remainssubstantially unchanged as the electrocatalytic layers remain adherentto the membrane and no bubbles are formed at the interfaces among themembrane and the electrocatalytic layers. In addition the CCM shows goodmechanical resistance in particular on the electrocatalytic layer edges.The three layers forming the CCM substantially show the same sizevariation in the two orthogonal directions of the plane xy as nodelamination takes place among the layers. The membrane used in thecomposite of the present invention is a (per)fluorinated ionomers basedmembrane and acts as an electrolyte.

Composites formed of membrnes having the surfaces coated with catalystsare known in the prior art. The composites are generally known with thename of Catalyst Coated Membrane or CCM. The two electrocatalytic layersof the CCM, usable in an electrochemical cell, form respectively thecell positive and the negative electrode.

By electrocatalytic layer it is meant a layer containing a metal andacts as electrode and as catalyst of the semireaction which takes placein the cell semielement wherein the electrode is placed.

The catalyst coated membrane can be used in polymeric electrolyte fuelcells and also in other electrochemical devices. In case of the fuelcell, when a fuel, for example hydrogen or methanol, is fed at thenegative electrode (anode), the fuel is oxidized by the catalyst in theelectrode, generating protons. The latter cross the negative electrode,the contact surface electrode/membrane and the membrane, reaching in theend the positive electrode (cathode). At this electrode an oxidizingagent, for example oxygen or air, is fed and in the presence of thecatalyst, reacts with the protons generating water. As a result of thereaction an electric current is generated and is obtained by connectingwith an electric wire the positive and the negative electrode. Thereforethis reaction can be used as energy source.

The possibility of supplying electric power depends on the entity of theproton flow from the anode to the cathode, and this depends on theresistance encountered by the protons crossing the anode, the interfacebetween the anode and the membrane, the membrane, the interface betweenthe membrane and the cathode and lastly the latter electrode. The higherthe flow, the more reduced these resistances.

In order to improve the proton flow, and thus to have high performances,it is essential to have a good contact between the electrodes and themembrane. To maintain high performances in the time, it is necessarythat the contact between the electrodes and the membranes be lasting.This requires that the CCM be dimensionally stable. In particular, thesize variations of the CCM after hydration must be the lowest aspossible to avoid that the electrodes detach under dynamic workingconditions. Dynamic working takes place, for example, when the deliveredcurrent is variable, or when turning on/turning off cycles are carriedout, or when the relative humidity levels of the fuel and/or oxidizingagent are variable. In these cases the amount of water present in thecell, and therefore the CCM hydration, significantly vary in the time.

The electrodes of the fuel cells polymeric electrolyte based aregenerally formed of a thin film, formed of catalyst particles (Pt or Ptalloys) finely dispersed on a conductive support, usually carbon powderand containing an ionomeric binder. Besides supplying consistency to theelectrode, the function of the ionomeric binder is to transport protons.At the anode, the flow takes place from the catalyst surface to themembrane one. At the cathode, the flow takes place from the membranesurface to the catalyst one.

In the preparation of the CCM the electrodes can be manufactured withdifferent methods. For example they can be directly prepared in contactwith the membrane. Alternatively they are initially placed on an inertsupport and then trnsferred on the surfaces of the ionomeric membrane.More in detail, the process for preparing the electrodes, used in theprior art, first comprises the preparation of liquid dispersionscontaining the catalyst, usually supported, together with the ionomericbinder. The dispersions are then deposited, directly on the membrane oron the inert support, depending on the considered preparation process.When the electrode layers are directly prepared on the membrane, thelatter is previously swolled with a solvent, preferably the same usedfor the preparation of the catalyst liquid dispersion, to avoid that itdeforms when is in contact with the dispersion. See for example U.S.Pat. No. 6,074,692 and U.S. Pat. No. 6,475,249. When the electrodelayers are prepared on an inert support and then transferred onto themembrane, as it occurs in the process called “decal”, the transfer iscarried out in hot conditions under pressure, for example attemperatures of the order of 150° C. and at pressures of 20 bar. See forexample U.S. Pat. No. 5,211,984 and U.S. Pat. No. 5,234,777. Thetransfer in hot conditions under pressure is a critical step of thisprocess. It has been found by the Applicant that the transfer in hotconditions under pressure can damage the membranes, especially whenthin.

Patent application US 2003/0219,532 describes a process for preparing aCCM consisting of various steps. In the first step a support is coatedwith a first polymeric electrolyte solution (ionomeric solution) to forma first polymeric membrane (ionomeric membrane), still containingsolvent at least on its surface. In the second step a first electrode orelectrocatalytic dispersion, comprising a second ionomeric solution anda catalyst, is placed on the ionomeric membrane. In the third step thefirst dispersion is dried to form the electrode-membrane assembly of thepositive electrode, formed by the ionomeric membrane and the firstelectrode. The assembly forms one of the two halves of the final CCM.The three steps are repeated by using another inert support, to obtainthe other CCM half, formed of the electrode-membrane assembly of thenegative electrode. In the process final step the two halves areseparated from the supports and are joined by interposing an ionomersolution, to favour the adhesion between the two membranes, to obtainthe final CM. This step has the drawback to cause size variations in thetwo membranes to be coupled. Besides, each of the two CCM halvesgenerally has a very low thickness. This causes the risk that during theassembly tearings or folds in the membrane occur. Tests carried out bythe Applicant have shown that the CCM prepared with this methoddeteriorates quickly when it is subjected to numerous heating andcooling cycles in water. The latter are used to simulate the conditionswhich take place during the working of a fuel cell. It has been foundthat the two parts forming the CCM tend to be detached.

U.S. Pat. No. 6,649,295 describes the preparation of electrode-ionomericmembrane assembly. The ionomeric membrane is obtained by casting from asolution and annealed at a temperature preferably higher than 120° C.The catalyst dispersion is deposited on a gas diffuser and attached tothe annealed membrane. The drawback of this process is that it does notguarantee a good contact between the electrode and the membrane.

The need was felt to have available a CCM having the followingcombination of properties:

-   -   reduced size variations during the hydration/dehydration cycles,        as it occurs for example during the cell turning on/turning off        steps or when the cell has to deliver powers variable in the        time, or also when the relative humidity levels of the fuel        and/or the oxidizing agent are varied;    -   improved adhesion of the electrocatalytic layer to the ionomeric        membrane;    -   substantially unchanged adhesion among the CCM layers also after        numerous dipping cycles in water at high temperature and then in        water at room temperature;    -   improved CCM duration in electrochemical cell, in particular        with improved resistance to the CCM breaks.

The Applicant has found CCM solving the above technical problem.

An object of the present invention is a CCM comprising a membrane havingon one of the two surfaces a first electrocatalytic layer containing acatalyst and on the other a second electrocatalytic layer containing acatalyst, wherein:

-   -   the orthogonal projection of the perimeter of the first        electrocatalytic layer on the second layer is distant from the        perimeter of the second electrocatalytic layer at leat 1 mm,        preferably 2 mm in each point of said projection;        the electrocatalytic layers and the membrane containing        (per)fluorinated ionomers, preferably sulphonic ionomers having        an equivalent weight in the range 380-1,700 g/eq, preferably        500-1,200 g/eq, the CCM having the following characteristics:    -   size variations, in the orthogonal directions of the plane xy,        lower than 15%, more preferably lower than 10%, measured after        CCM dipping in demineralized water at 100° C. for 30 minutes,        dried at 105° C. under vacuum for one hour;    -   the CCM remains substantially unchanged after 60 cycles by        dipping in water at 80° C. for 8 hours and then in water at the        temperature of 25° C. for 16 hours, as the CCM electrocatalytic        layers remain adherent to the membrane and no bubbles form at        the interfaces between the membrane and the electrocatalytic        layers.

As said, the perimeter projection of the first electrocatalytic layer onthe second one is distant from the perimeter of the secondelectrocatalytic layer at least 1 mm, preferably 2 mm in each point ofthe projection. A maximum value of said distance does not exist, it isgenerally preferable that it is not higher than 3 mm to avoid that asignificant part of the electrocatalytic layer surface remains unusedduring the utilization in electrochemical cell. It has been found by theApplicant that, by using the CCM of the invention in an electrochemicalcell, no CCM breaks take place, in particular at the edges of theelectrocatalytic layers.

In the CCM according to the present invention the two electrocatalyticlayers and the membrane are clearly distinct among each other and thecatalyst is substantially absent from the membrane.

The above CCM is obtainable with the process described below.

The (per)fluorinated ionomers usable for preparing the membranes and theelectrocatalytic layers of the CCM of the present invention areobtainable from ionomers having the following units:

-   -   copolymers comprising:        -   (A) monomeric units deriving from one or more fluorinated            monomers containing at least one ethylenic unsaturation;        -   (B) fluorinated monomeric units containing sulphonyl —SO₂F            groups in amounts such that the ionomer has the equivalent            weight in the above range; or    -   homopolymers formed of monomeric units (B); by hydrolysis of the        —SO₂F groups to obtain the sulphonic groups in acid —SO₃H or        salified form.

-   The fluorinated monomers (A) are selected from:    -   vinylidene fluoride (VDF);    -   C₂-C₈ perfluoroolefins, preferably tetrafluoroethylene (TFE);    -   C₂-C₈ chloro- and/or bromo- and/or iodo-fluoroolefins, as        chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;    -   CF₂═CFOR_(f1) (per)fluoroalkylvinylethers (PAVE), wherein R_(f1)        is a C₁-C₆ (per)fluoroalkyl, for example trifluoromethyl,        bromodifluoromethyl, pentafluoropropyl;    -   CF₂═CFOX perfluoro-oxyalkylvinylethers, wherein X is a C₁-C₁₂        perfluoro-oxyalkyl having one or more ether groups, for example        perfluoro-2-propoxy-propyl.

-   The fluorinated monomers (B) are selected from one or more of the    following:    -   F₂C═CF—O—CF₂—CF₂—SC₂F;    -   F₂C═CF—O—[CF₂—CX_(A)F—O]_(nA)—(CF₂)_(nB)—SO₂F    -   wherein X_(A)═Cl, F or CF₃; nA=1-10, nB=2, 3;    -   F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F;    -   F₂C═CF—Ar—SO₂F wherein Ar is an aryl ring.

Optionally the sulphonic fluorinated ionomers of the invention cancontain from 0.01% to 2% by moles of monomeric units deriving from abis-olefin of formula:R₁R₂C═CH—(CF₂)_(m)—CH═CR₅R₆   (I)

-   wherein:-   m=2-10, preferably 4-8;-   R₁, R₂, R₅, R₆, equal to or different from each other, are H or    C₁-C₅ alkyl groups.

Other usable monomers are those containg precursor groups which byhydrolysis are transformed into acid —COOH groups or correspondingsalts; said monomers can be used in the place of —SO₂F containingmonomers, or in admixture with those containing —SO₂F groups.

The fluorinated monomers (B) used to prepare the ionomers containingacid —COOH groups have the following structures:

-   -   F₂C═CF—O—CF₂—CF₂—Y;    -   F₂C═CF—O—[CF₂—C_(A)F—O]_(nA)—(CF₂)_(nB)—Y    -   wherein X_(A)=Cl, F or CF₃; nA=1-10, nB=2, 3;    -   F₂C═CF—O—CF₂—CF₂—CF₂—Y;    -   F₂C═CF—Ar—Y wherein Ar is an aryl group;        wherein Y is a precursor group of the carboxylic group, selected        from the following: CN, COF, COOH, COOR_(B), COO⁻Me⁺,        CONR_(2B)R_(3B), wherein R_(B) is C₁-C₁₀, preferably C₁-C₃ alkyl        and R_(2B) and R_(3B), equal or different, are H or have the        meaning of R_(B), Me is an alkaline metal.

As said, the fluorinated monomers (B) with end group Y having the aboveformulas can be in admixture with the fluorinated monomers containingsulphonyl groups —SO₂F, the total amount of the monomers (B) being suchthat the equivalent weight of the ionomer is in the above range.

Preferably the membranes and the electrocatalytic layers of the CCM ofthe present invention contain perfluorinated ionomers obtainable fromionomers comprising:

-   -   monomeric units deriving from TFE;    -   monomeric units deriving from CF₂═CF—C—CF₂CF₂SO₂F.

The hydrolysis of the precursors of acid functional groups of theionomers comprises two steps; the first is carried out in basicenvironment and the second in acid environment, obtaining the ionomerswith functional groups in the acid —SO₃H and/or —COOH form.

For example, in case of sulphonyl precursor groups —SO₂F, they aretransformed into sulphonic groups —SO₃H by the following steps:

-   -   salification of the —SO₂F form into the —SO₃ ⁻Me₊ form, where Me        is an alkaline metal;    -   acidification of the —SO₃ ⁻Me⁺ form into the —SO₃H form.

The first step can for example be carried out by mixing the ionomericpolymer with an aqueous solution containing 10% by weight of KOH, at atemperature between 60° C. and 80° C., for a time higher than 2 hours,until disappearance of the —SO₂F groups (IR analysis) and formation ofthe —SO₃ ⁻Me⁺ group. At the end of the salification step, the ionomer iswashed with water at a temperature preferably not higher than 25° C. Theacidification step is carried out, for example, by transferring thesalified ionomer into an aqueous solution containing 20% by weight ofHCl at room temperature and by keeping under stirring for at least halfan hour. At the end a washing with water is carried out according to theabove method.

The ionomeric polymers are available on trade. Those known with thetrademark Nafion® can for example be mentioned.

The membranes and the electrocatalytic layers of the CCM of the presentinvention are prepared by starting from solutions and/or dispersions ofthe ionomers. See for example EP 1,004,615 and U.S. Pat. No. 4,433,082.

The membranes and the electrocatalytic layers of the CCM of the presentinvention have a thickness ranging from 3 micrometers to 100micrometers. The membranes preferably from 10 to 80 micrometers, morepreferably from 15 to 60 micrometers, the electrocatalytic layerspreferably from 5 to 50 micrometers, more preferably from 5 to 30micrometers.

The electrocatalytic layers comprise an ionomer and a catalyst,preferably Pt or a mixture of Pt with one or more metals, as, forexample, Ru, Rh, Mo. Said catalyst is finely dispersed and preferablysupported on carbon powder. The powders known with the followingcommercial names can for example be used: Vulcan XC-72, Ketjen Black,Black Pearls, Shawinigan Acetylene Black, etc. The ionomer hascomposition and/or equivalent weight equal to or different from theionomer used in the membrane and/or in the other electrocatalytic layer.The ratio by weight between catalyst and ionomer in each of the twoelectrocatalytic layers ranges from 0.5 to 4, preferably from 0.5 to2.5. The ratio by weight between the catalyst and the powder support ispreferably higher than or equal to 10; when as fuel, hydrogen is used,the ratio is between 20 and 60, when methanol is used between 60 and100.

The mg of catalyst metal/cm² of electrocatalytic layer ratio ranges from0.01 to 2; when in the cell hydrogen is used as fuel, the ratiopreferably ranges from 0.01 to 0.7 mg/cm², preferably by using at thecathode side a ratio ranging from 0.1 to 0.7 mg/cm²; when methanol isused as fuel, said ratio preferably ranges from 0.3 to 1 mg/cm² at theanode side and from 0.5 to 2 mg/cm² at the cathode side.

A further object of the present invention is a process for preparing CCMaccording to the present invention comprising the following steps:

-   1) formation of the first electrocatalytic layer on the surface face    of an inert support with the following steps:    -   1a) preparation of a first liquid dispersion comprising an        ionomer, in acid or salified form, and a catalyst, as defined        above, wherein the ionomer concentration (in % by w.) ranges        from 0.5% to 40%, preferably from 0.5% to 25%, the ratio by        weight between catalyst and ionomer being from 0.5 to 4;    -   1b) the dispersion prepared in 1a) is placed on the surface of        an inert support;    -   1c) removal of the solvent for at least 80% by weight with        respect to the initial solvent of the dispersion, until        obtaining a first electrocatalytic layer having:        -   an extractable ionomer percentage, determined by the test            described hereinafter, ≧80% by weight, even up to 100% with            respect to the total ionomer present in the layer;        -   a catalyst amount of electrocatalytic layer between 0.01 and            2 mg/cm²;-   2) on the first electrocatalytic layer obtained in step 1),    formation of the ionomeric membrane through the following steps:    -   2a) preparation of a liquid dispersion of an ionomer in acid or        salified form, equal to or different from that used in step 1a)        and having a concentration, (% by w.) on the total dispersion        weight, from 3% to 40%, preferably from 7% to 25%;    -   2b) placing the liquid dispersion 2a) on the free surface of the        first electrocatalytic layer;    -   2c) solvent removal for at least 80% by weight as in 1c),        obtaining an ionomeric membrane on the top of the first        electrocatalytic layer; the ionomer percentage extractable after        this step being ≧80% by weight, even up to 100% with respect to        the total ionomer present in the first catalytic layer and the        membrane;-   3) formation of the second electrocatalytic layer on the surface of    the ionomeric membrane free from catalyst as obtained in step 2), to    obtain the raw CCM on the inert support through the following steps:    -   3a) preparation of a second liquid dispersion comprising an        ionomer, in acid or salified form, and a catalyst, whereien the        ionomer concentration is (% by w.) between 0.5% and 40%,        preferably between 0.5% and 25%, the catalyst/ionomer ratio (%        by w.) being between 0.5 and 4; in said second dispersion the        ionomer and/or the catalyst being equal to or different from        those used in 1a) and in 2a);    -   3b) deposition of the liquid dispersion 3a) on the surface of        the ionomeric membrane free from the catalyst prepared in 2) to        form the second electrocatalytic layer positioned on the        membrane so that the orthogonal projection of its perimeter on        the first electrocatalytic layer is distant from the perimeter        of the latter at least one millimeter, preferably two        millimeters in each point of the projection;    -   3c) solvent removal for at least 80% by weight with respect to        the initial solvent of the dispersion used, as in 1c), until        obtaining the second electrocatalytic layer, having:        -   a catalyst amount as indicated in 1c);

the ionomer percentage extractable after this step from the raw CCM(membrane+first and second electrocatalytic layer), determined with thetest described hereinafter, being ≧80% by weight, even up to 100% withrespect to the total weight of the ionomer present in the CCM;

-   4) annealing of the system (raw CCM supported on the inert support)    at temperatures from 120° C to 280° C. and obtainment of the CCM by    separation of the inert support from the CCM.

In steps 1c) and 2c), as said, the solvent must be removed for at least80% by weight (with respect to the initial amount), but it is preferablethat some solvent remains present. In this way after the step 1c) and2c) non consolidated systems are obtained. This means that the systemplaced in water/ethanol (60/40% by weight) at 50° C. for 22 hours, therewould dissolve almost completely. Residual amounts of solvent up to 5%by weight with respect to the starting one are preferred.

Step 3c) can be carried out as steps 1c) and 2c) as regards the amountof residual solvent. Alternatively, in step 3c) the solvent can beremoved even up to 100%.

When in the preparation of at least an electrocatalytic layer and/or ofthe membrane, as ionomers, copolymers containing units deriving from thesulphonic monomer (SSC) CF₂═CF—O—(CF₂)₂SO₂F are used, the annealing ofstep 4) ranges from 130° C. to 280° C., preferably from 140° C. to 180°C., still more preferably from 140° C. to 160° C.

The percentage of extractable ionomer (by w.) is determined by dippingthe sample, obtained at the end of the steps 1), 2), 3), in awater/ethanol mixture (the alcohol being 40% by weight) and maintainingthe temperature of the mixture at 50° C. for 22 h, filtering on Whatmann541 filter, drying the liquid phase filtered, drying the residue up to80° C. until a constant weight. When the catalyst is present, athermogravimetric analysis is carried out on the residue. Being knownthe amounts of the ionomer and of the catalyst, extractable ionomerpercentage by w. is calculated.

As inert support it is meant a material substantially remainingchemically and physically unchanged under the conditions used forpreparing the CCM. The inert support usable in step 1) can be anysupport preferably non porous and with smooth surfaces. Preferably ithas a size variation in each of the two directions of the plane xy,measured at the temperature of step 4) for 15 minutes, not higher than2%, preferably not higher than 1%. More preferably the support maintainssubstantially unchanged its properties at the temperatures of step 4).The support has shown an easy detachment of the CCM. The skilled in theart is able to determine the suitable support on the basis of thetemperature of step 4). For example, the support can be non porous PTFE,polyimide, for example marketed with the trademark Kapton®, MFA, PFA,polyesters such as PET.

As said, the ionomer dispersions used in the process, can be preparedaccording to known techniques in the field. In steps 1a), 2a) and 3a),the solvent is selected from C₁-C₃, preferably C₃, alcohols, n-propanoland/or iso-propanol, or mixtures, preferably with water, of C₁-C₃alcohols. Optionally other organic solvents can be used, provided thatthey are miscible with water and/or with the above alcohols. Examples ofoptional solvents are the following: dimethyl sulphoxide (DMSO), ethylenglycol (EG), N,N′-dimethylformamide (DMF), triethyl phosphate (TEP),2-ethoxy-ethanol (2EE), N,N′-dimethylacetamide (DMA),N-methylpyrrolidone (NMP), acetonitrile (AN) and propylencarbonate (PC),fluoropolyoxyalkenes having one hydrogen atom at one or at both chainends; the fluoropolyoxyalkenes preferably have boiling point between 80°C. and 120° C.

The dispersion, besides the ionomer and the catalyst, can contain othercomponents, as (per) fluoropolymer fillers; inorganic fillers as, forexample, silica, Zr or Ti acid phosphates, surfactants.

In steps 1a) and 3a), the dispersion is preferably prepared first bydissolving or dispersing the ionomer in the solvent and then dispersingthe catalyst. This order of addition of the components of the dispersionhas the advantage that a final dispersion sufficiently stable isobtained, without separation of the catalyst.

The deposit of the dispersions in steps 1b), 2b) and 3b) is carried outaccording to known techniques. For example spray coating, casting withknife, kiss-coating, serigraphy, ink-jetting, curtain coating, etc canbe mentioned.

In step 1b) the deposit of the first electrocatalytic or electrodiclayer can be carried out as illustrated in FIG. 1, wherein 1 is thefirst electrodic layer and 2 is a smooth support. In this case, in thefinal CCM the first electrodic layer is at least partially englobed inthe membrane.

Alternatively, in step 1b) the deposit of the first electrocatalytic orelectrodic layer is carried out as illustrated in FIG. 2 b on a support3 containing an impression corresponding to the form of the firstelectrodic layer 4, removing the excess of the dispersion 1a); theheight of the first electrodic layer, after solvent evaporation (step1c)), must be equal to the heigth 5 of the impression (FIG. 2).Therefore, by using this deposition method, in the final CCM the firstelectrodic layer is outside the membrane.

Preferably the obtainment of the first electrocatalytic layer in step 1)of the process of the present invention is carried out, see FIG. 3,through the following steps:

-   -   on the support 2 is placed a thin and flat sheet or mask 6,        wherein a window having the perimeter of a regular flat figure        is cut out, for example a parallelogram, corresponding to the        form of the electrodic layer;    -   deposit of the dispersion prepared in 1a) (step 1b) in the        window of the thin sheet, removing the excess of the dispersion        by a blade running in contact with the surface of the sheet 6;    -   solvent removal (step 1c);    -   removal of the sheet 6.

The height of the sheet 6 is selected so to obtain a catalystamount/surface unit ratio of the electrodic layer in the above range.

In step 2a) the ionomeric dispersion used for forming the membrane has asufficiently high viscosity, generally higher than 100 cP (measured at25° C. and at a shear rate of 0.1 sec⁻¹), preferably higher than 500 cP.A sufficiently high viscosity avoids, during the membrane deposition,that the first not consolidated electrodic layer is accidentally removedand that the dispersion used for the membrane deposition penetratesinside the pores of the first electrodic layer.

In step 2b) the membrane deposit is preferably carried out with acontinuous process of solutions or dispersions, as casting with knife,curtain coating, etc.

The same above described process for obtaining the first electrodiclayer using the sheet or mask 6 can be repeated to obtain the secondelectrodic layer according to the steps 3a)-3c), by applying in thiscase the sheet 6 on the uncoated surface of the membrane. Preferably, inthis case, mask 6 is removed before the solvent removal step 3c). Assaid above, the window cut out from sheet 6 has shape and sizes and ispositioned on the membrane so that the orthogonal projection of itsperimeter on the first electrocatalytic layer is distant from theperimeter of the latter at least one millimeter, preferably twomillimeters in each point of the projection.

The obtainment of the CCM as indicated in FIG. 3, wherein the mask 6 isused for building the electrodes, can preferably be carried out in acontinuous way.

In FIG. 4 the transversal section of the CCM is represented, 7 and 8being the electrocatalytic layers and 9 the membrane. It is noted thatthe orthogonal projection of the edges of the electrocatalytic layer 8falls outside the edges of the electrocatalytic layer 7. In FIG. 4 b thesame CCM is represented, seen from the top. The edges of theelectrocatalytic layer 7, placed underneath the membrane and thereforenot visible, are represented by a discontinuous line.

In steps 1c), 2c), 3c) the solvent removal is carried out by thermaltreatment at 45°-95° C., by preferably operating at atmosphericpressure, until obtaining a content of residual solvent in the abovelimits. For example by operating at a temperature of 65° C. a time of30-45 minutes is required.

After the step 3c), one passes to step 4), preferably by using a thermalgradient of about 10° C. in the time of 3-30 minutes, preferably 10-20minutes. A thermal gradient of 1° C./-minute is generally used.

It has been found by the Applicant that the steps 1c), 2c) 3c) and thethermal treatment 4) are essential for obtaining and maintaining in thetime, under the use conditions, the CCM integrity and to have a high CCMendurance. The latter feature is in particular obtained if the abovethermal gradient is used.

The time employed in step 4) depends on the thermal treatmenttemperature. Usually, a time generally higher than 15 minutes and lowerthan 10 hours is required. For example, by operating at 170° C., timesbetween about 30 minutes and 1 hour are generally sufficient.

The separation of the CCM from the support can be carried out in dryconditions, or by dipping in water, generally at temperatures in therange 20°-25° C.

The process of the present invention shows the following advantagescompared with those of the prior art for preparing CCM:

-   -   in all the steps the layers forming the final CCM are supported,        therefore the drawbacks occurring when coupling unsupported        membranes are eliminated;    -   in the preparation of the CCM the membrane is deposited on the        first electrocatalytic layer and the second electrocatalytic        layer is deposited on the membrane after evaporation of the        solvent in the indicated percentages but the extractable ionomer        percentage being higher than or equal to 80%. In this way it has        been found that an optimal adhesion among the various interfaces        is obtained and thus the integrity of the CCM is obtained under        the use conditions, even in the most drastic ones, for prolonged        times;    -   when the membrane, obtained at the end of step 2c), is contacted        with the dispersion of the catalytic layer in step 3b), it does        not show any size variation;    -   the three layers forming the CCM show the same size variation in        the two orthogonal directions of the plane xy as there is no        delamination among the layers;    -   the membrane and the two electrode layers are contemporaneously        annealed in step 4), thus creating an optimal adhesion and        lasting in the time among the CCM layers; the CCM obtained by        annealing on the support gets a high dimensional stability;    -   by carrying out the evaporation of the solvent at the end of        each single step of the process, before the deposit of the        successive layer, the catalyst remains in the electrocatalytic        layer and there is no substantial migration thereof into the        membrane layer. This has the advantage that the catalyst is        wholly used for the electrocatalytic reaction. This brings to        improved CCM compared with the cases wherein the catalyst        migrates inside the membrane. From the industrial point of view,        this represents a remarkable advantage as the catalyst has a        significant incidence on the total cost of the manufactured        article.

By using the method of the present invention a CCM is obtained whereinthe electrode-membrane contact is optimal. The Applicant has found thatthe direct deposition of the second catalytic layer on the membranepreviously annealed brings to the obtainment of CCM having poorproperties as a unhomogeneous and discontinuous catalytic film isformed.

With the process of the invention the hot transfer and under pressuresteps of the catalyst on the membrane, used in the prior art forobtaining the CCM (decal process), are eliminated. It has been found bythe Applicant that these steps negatively affect the performances incell and the duration of the CCM.

The preparation of the ionomers can be carried out with a mass,suspension, emulsion radical polymerization process.

The aqueous emulsion or microemulsion polymerization can for example bementioned. The surfactants usable in these polymerizations are(per)fluorinated, for example salts (as defined below) of theperfluorooctanoic, perfluorononanoic, perfluorodecanoic acid, or theirmixtures, etc., (per)fluoropolyethers with one acid end group (forexample COOH, SO₃H), salified with NH₄ ⁺ or with alkaline metal cations,the other end group being (per)fluorinated, optionally containing one Hor Cl atom. The number average molecular weights of theperfluoropolyether surfactants generally range from 300 to 1,800,preferably from 350 to 750. The microemulsion polymerization is wellknown in the art. In particular the ionomer preparation is carried outby using an aqueous emulsion wherein, in the reaction medium, assurfactants, those of formula:R_(f)—X₁ ⁻M⁺are used, wherein:

-   -   X₁ is equal to —COO, —SO₃;    -   M is selected from H, NH₄ or an alkaline metal;    -   R_(f) represents a (per)fluoropolyether chain, preferably having        number average molecular weight in the range from about 230 to        about 1,800, preferably from 300 to 750, said        (per)fluoropolyether chain comprising repeating units selected        from one or more of the following:    -   a) —(C₃F₆O)—;    -   b) —(CF₂CF₂O)—;    -   c) —(CFL₀O)—, wherein L₀=—F, —CF₃;    -   d) —CF₂(CF₂)_(z), CF₂O—, wherein z′ is an integer 1 or 2;

e) —CH₂CF₂CF₂O—.

R_(f) is monofunctional and has a (per)fluorooxyalkyl end group T, forexample CF₃O—, C₂F₅O—, C₃F₇O—; optionally in perfluoroalkyl end groupsone fluorine atom can be substituted with one chlorine or hydrogen atom.Examples of these end groups are Cl(C₃F₆O)—, H(C₃F₆O)—. The unit a)C₃F₆O is —CF₂—CF(CF₃)O— or —CF(CF₃)CF₂O—.

In the above formula R_(f) preferably has one of the followingstructures:

-   1) T-(CF₂O)_(a)—(CF₂CF₂O)_(b)—CF₂—    -   b, a being integers,    -   with b/a between 0.3 and 10 where a is different from 0,        extremes included, a being an integer different from 0;-   2) T-(CF₂—(CF₂)_(z′)—CF₂O)_(b′)—CF₂—    -   b′ and z′ being integers,    -   wherein z′ is an integer equal to 1 or 2;-   3) T-(C₃F₆O)_(r)—(C₂F₄O)_(b)—(CFL₀O)_(t)—CF₂—    -   r, b, t, being integers,    -   with r/b ranging from 0.5 to 2.0 where b is different from 0;        (r+b)/t ranges from 10 to 30, where t is different from 0;        a, b, b′, r, t are integers, their sum is such that R_(f) has        the above values of number average molecular weight.

The compounds wherein R_(f) has the following formula:T-(CF₂CF(CF₃)O)_(m)(CF₂O)_(n)—CF₂—

-   m, n being integers;-   m/n ranges from 1 to 30;-   wherein T=—OCF₃ or —OCF₂Cl,-   are still more preferred.

The (per) fluoropolyethers R_(f) are obtainable with the well knownprocesses in the prior art, see for example the following patents hereinincorporated by reference: U.S. Pat. No. 3,665,041, U.S. Pat. No.2,242,218, U.S. Pat. No. 3,715,378 and the European patent 239,123. Thefluoropolyethers functionalized with hydroxyl termination are obtainedfor example according to EP 148,482, U.S. Pat. No. 3,810,874. The endfunctional groups are obtained with the processes indicated in saidpatents.

Chain transfer agents can be used in the polymerization. For examplealkaline or earth-alkaline metal iodides and/or bromides, according toU.S. Pat. No. 5,173,553. Chain transfer agents containing hydrogen, ashydrocarbons, alcohols, in particular ethyl acetate and ethane arepreferably used.

The polymerization initiators used in the process of the presentinvention are preferably radical inorganic initiators, as, for example,ammonium and/or potassium and/or sodium persulphate, optionally incombination with ferrous, cuprous or silver salts. The procedures of theinitiator feeding into the polymerization reactor can be in a continuousway or by a single addition at the beginning of the polymerization.

The polymerization reaction is generally carried out at temperatures inthe range 25° C.-70° C., preferably 50° C.-60° C., under pressure up to30 bar (3 MPa), preferably higher than 8 bar (0.8 MPa).

Monomer (B) is fed into the poylmerization reactor in a continuous wayor by steps.

After the polymerization is completed, the ionomer is isolated byconventional methods, as the coagulation by addition of electrolytes orby freezing.

The CCM of the present invention can be used in electrochemical cells,in particular in fuel cells.

The following Examples are given for illustrative and not limitativepurposes of the present invention.

EXAMPLES

Characterization

Determination of Equivalent Weight

About 1 gram of the polymer is dried at 150° C. for 40 h. From the driedpowder a thin film of about 100 μm is obtained by molding in press atthe temperature of 280° C. The so obtained film is treated at 80° C. for24 h with KOH at 10% by weight, then washed with demineralized water andthen treated at room temperature for 24 h with HCl at 20% by weight. Atthe end it is washed with demineralized water. In this way the sulphonylgroups of the film are converted into acid sulphonic groups.

The polymer film in acid form is dried at 105° C. up to a constantweight and weighed; then the film is suspended in a hydroalcoholic or anaqueous solution, an excess of a titrated NaOH solution is added and itis titrated back with a HCl titrated solution. The equivalent weight isdetermined by the ratio between the weight of the film, expressed ingrams, and the number of equivalents of titrated acid groups.

Determination of the Performance in Fuel Cell

The CCM is assembled between two gas diffusion layers ELAT®/nc/ss/V2(E-TEK, Inc.) having 10 cm² area; the cell is fed from the anode sidewith hydrogen and from the cathode side with air. The gas feedingpressure to the cell is equal and is 0.25 MPa. The cell temperature ismaintained at 75° C. and the feeeding gases are previously saturatedwith water at 80° C.

By a load applied to the circuit outside the cell, the current intensityis stabilized maintaining the voltage at 0.4 Volt. Then the currentintensity (current per surface unit of electrode) is regulated and thevoltage is measured at the two poles of the cell. The operation isrepeated by using different external loads and the voltage obtained atdifferent current intensities in the cell is determined. In particularthe voltage obtained at 0.2 and 1 A/cm² is determined.

Evaluation of the Integrity of the CCM after Prolonged Contact withWater

The test is indicative of the integrity of the CCM under the useconditions in fuel cell.

The CCM is dipped in demineralized water at the temperature of 80° C.for 8 hours. The CCM is then transferred into demineralized water atroom temperature for 16 hours. The procedure is repeated for 60 times inall. At the end of the test the integrity of the CCM is examined for thepossible detachment of one or more layers, and for the presence ofcracks or bubbles in correspondence with the interfaces among thelayers.

The CCM, if it appears integral, is left dipped in water, at roomtemperature, and one stirrs with a glass rod for two minutes andobserves if during this time the possible detachment of layers takesplace.

Determination of the Size Variations of the CCM in the Two OrthogonalDirections of the Plane

The CCM is cut out so as to obtain a squared membrane, having 7×7 cmside with the electrocatalytic layer in central position and having3.2×3.2 cm sizes.

The CCM is initially dried at 105° C. under vacuum for 1 hour. The CCMmembrane sizes are determined. The CCM is then hydrated withdemineralized water at 100° C. for 30 minutes; after removal from waterit is determined how much the membrane has stretched in the two planedirections.

The size variations in the two orthogonal directions are calculated aspercent referred to the starting sizes, after drying at 105° C. undervacuum for 1 hour.

Determination of the Extractable Ionomer

The extractable ionomer is determined by dipping the piece, for examplethe membrane, in a water/ethanol mixture containing 40% by wiehgt ofalcohol and maintaining the mixture temperature at 50° C. After 22 h onefilters on Whatman filter 541 and the filtered liquid phase is dried. Itis dried at 80° C. up to constant weight, obtaining the total amount ofionomer extracted. This amount, divided by that of the ionomer presentin the piece, gives the percentage of the extractable ionomer,solubilized under the conditions used in the test.

In case where the ionomer is extracted from a piece of the CCMcomprising at least an electrocatalytic layer, on the obtained residue athermogravimetric analysis is carried out to determine the amount ofcatalyst present. The final temperature of the thermogravimetricanalysis is 600° C. The weight of the obtained residue corresponds tothat of the catalyst present.

Determination of the Viscosity of the Dispersion

The viscosity (Brookfield) is determined by a viscometer SynchroElectric® LVT model, measured at 25° C. with rotor No. 4 at 60 rpm.

Example 1 Preparation of the Ionomer in SO₃H Form

In a 22 litre autoclave the following reactants are introduced:

-   -   11.5 litres of demineralized water;    -   980 g of the monomer of formula CF₂═CF—O—CF₂CF₂—SO₂F;    -   3,100 g of an aqueous solution at 5% by weight of a        fluoropolyoxyalkylene having acid end group with number average        molecular weight 521 potassium salified, of formula:        CF₂ClO(CF₂CF(CF₃)O)_(n)(CF₂O)_(m)CF₂COOK        wherein n/m=10.

The autoclave, kept under stirring at 540 rpm, is heated to 60° C. Then225 ml of an aqueous solution having a concentration of 6 g/l ofpotassium persulphate (KPS) are fed into the autoclave. The pressure isbrought to 1.3 MPa absolute by introducing TFE. The reaction startsafter 4 min. The pressure is maintained at 1.3 MPa absolute by feedingTFE. After 1,000 g of TFE have been fed into the reactor, 175 g of thesulphonic monomer of formula CF₂═CF—O—CF₂—CF₂—SO₂F are introduced. Then175 g of the same sulphonic monomer are introduced in the reactor every200 g of TFE fed. The total TFE mass fed to the reactor is 4,000 g.

The reaction is stopped after 233 minutes by interrupting the TFEfeeding, cooling and venting the reactor under vacuum. The producedlatex has a solid content of 28.5% by weight. The latex is coagulated byfreezing and defrosting, the polymer separated from the mother liquors,washed with water up to constant pH of the washing waters.

The equivalent weight of the copolymer is 870 g/eq, corresponding to acomposition 85.56 molar of TFE and 14.5% molar of sulphonic monomer.

A part of the polymer is separated from the washing waters and istreated with a 20% by weight KOH solution at 80° C. for 6 hours, bykeeping under stirring. For one part by weight of polymer 10 parts byweight of KOH solution are charged. At the end it is washed withdemineralized water up to stable pH of the washing waters 10 parts byvolume are added for one part by weight of polymer of a 20% HClsolution, maintaining under stirring at room T for 2 h. At the end it iswashed with demineralized water up to stable pH of the washing waters.The addition steps of the HCl solution and of the subsequent washingwith water are repeated for other two times. At the end the polymer inthe SO₃H form is recovered and dried for 40 h at 80° C.

Example 2 Preparation of the Dispersion of the Ionomer

An 8.6% by weight dispersion of the sulphonic ionomer obtained in theExample 1 is prepared by dissolving 18.8 g of ionomer in 200 g of aquaternary mixture composed of 39 g of H₂O, 80 g of isopropanol, 80 g ofn-propanol and 1 g of a fluoropolyoxyalkylene having formula:CF₂H—O(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₂Hwith boiling range between 80° C. and 120° C. and number averagemolecular weight equal to 350. The dissolution is carried out at roomtemperature in 24 hours in a glass vessel equipped with mechanicalstirrer. At the end the dispersion has a viscosity of 1,000 cP.

Example 3 Preparation of an Ionomer Dispersion and Catalyst

A 3% by weight dispersion of the sulphonic ionomer obtained in theExample 1 is prepared by dissolving 3.1 g of ionomer in 100 g of aquaternary mixture composed of 19.5 g of H₂O, 40 g of isopropanol, 40 gof n-propanol and 0.5 g of a fluoropolyoxyalkylene having formula:CF₂H—O(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₂Hwith boiling range between 80° C. and 120° C. and number averagemolecular weight equal to 350. The dissolution is carried out at roomtemperature in 24 hours in a glass vessel equipped with mechanicalstirrer.

To the so obtained ionomer dispersion, maintained under stirring bymagnetic stirrer, 26.9 g of Pt catalyst are added, under nitrogen flow,supported on Vulcan XC 72 (E-Tek) carbon, containing 30% by weight of Ptwith respect to the carbon. The ratio by weight platinum/ionomer is 2/1.

Example 4 Preparation of a CCM According to the Invention

To a non porous PTFE support (ref. 2 in FIG. 3) previously weighed aKapton mask (6 in FIG. 3) is applied having a thickness of 50 μm and inthe middle a square blank space of 10 cm² area (side 3.2 cm).

Along the whole length of one side of the blank space, a strip of thedispersion of the Example 3 is deposited on the mask to form theelectrocatalytic layer. With a stratifying Braive® knife set at knifeheight zero, placed parallelly to the considered side of the blank spaceand which moves perpendicularly to it, the dispersion is spread on thewhole surface of the blank space of the mask, so as to fill it.

It is transferred into an oven at 65° C. for 30 minutes to evaporate thesolvent. At the end it is extracted from the oven and the mask isremoved obtaining on the support the first electrocatalytic layer incorrespondence of the blank space of the mask. It is weighed and, on thebasis of the area of the electrocatalytic layer, it is found that theplatinum amount of the electrocatalytic layer is equal to 0.35 mg/cm².

The ionomeric membrane of the CCM is prepared by initially depositing onthe support, parallelly to a side of the first electrocatalytic layer, astrip of the dispersion of the Example 2, having a length of about 10 cmand distant about 4 cm from the considered side of the electrocatalyticlayer. The deposition is carried out so that the strip results nearlyequally protruding with respect to the edges of the considered side ofthe electrocatalytic layer. With a stratifying Braive® knife, set atknife height 1,100 μm, placed parallelly to the side of theelectrocatalytic layer, and which is moved perpendicularly to said side,the dispersion is spread so as to coat the first electrocatalytic layerand over, up to a distance of at least 4 cm with respect to the sideparallel to that considered of the electrocatalytic layer. It is driedin a stove at 65° C. for 30 minutes evaporating the solvent. In this waythe ionomeric membrane is formed over the first electrocatalytic layer.

On the free surface of the so obtained ionomeric membrane a Kapton® mask(polyimide) with a thickness of 50 μm is placed, having in the middle asquare blank space of 13 cm² area (side 3.6 cm). The mask is positionedso that the blank space is centred with respect to the firstelectrocatalytic layer.

The above process is repeated to obtain the first electrocatalytic layeruntil the deposition of the dispersion of the Example 3 with astratifying Braive® knife. The mask is taken off and it is transferredinto the oven at 65° C. for 30 minutes to evaporate the solvent. Thenthe temperature is gradually increased (10° C./10 min) up to 150° C.(annealing temperature). The second electrocatalytic layer is in thisway obtained on the free surface of the ionomeric membrane, and thus theraw CCM, which is supported on the PTFE support.

The temperature of 150° C. is maintained for 90 minutes so as to carryout the annealing phase. At the end it is taken out from the oven andthe PTFE support is removed obtaining the CCM according to the presentinvention.

The disposition of the two electrocatalytic layers is illustrated inFIGS. 4 and 4 b, wherein 7, 8 show the electrocatalytic layers and 3indicates the membrane.

Determination of the Percentage of the Extractable Ionomer

The preparation of the first electrocatalytic layer is repeated. It isfound that the percentage of the extractable ionomer is higher than 95%.

The preparation process of the first electrocatalytic layer+ionomericmembrane as described above is repeated. It is found that the percentageof the extractable ionomer is higher than 95%.

The preparation process of the first electrocatalytic layer+ionomericmembrane+second electrocatalytic layer as described above is repeated,without carrying out the annealing step. It is found that the percentageof the extractable ionomer is higher than 95%.

The extractable ionomer amount determined on the CCM after final thermaltreatment is lower than 10%.

Example 4a Performance in Fuel Cell of the CCM of the Example 4

The CCM of the previous Example is subjected to the above described testin fuel cell. The obtained results are the following: at 0.2 A/cm² thevoltage is 0.81 V and at 1 A/cm² the voltage is 0.66 V. At the end ofthe test it is noted that, disassembling the cell, the structuralintegrity of the CCM is maintained.

Example 4b Integrity Test of the CCM of the Example 4 by ProlongedContact with Water

The CCM of the Example 4 is subjected to the above test of prolongedcontact with water. At the end of the test the CCM appears integral,even after having left the CCM in water, at room temperture, and havingstirred with a glass rod for two minutes. In particular it is notobserved the presence of cracks or bubbles in correspondence of theinterfaces among the layers.

Example 4c Size Variations of the CCM of the Example 4

The CCM of the Example 4 is subjected to the above test to determine thesize variations. At the end of the test the dimensional variations inthe two orthogonal directions are the same and equal to 8%.

Example 5 (Comparative) Preparation of a CCM According to the Prior Artby the Process Called “Decal”

According to this process each electrocatalytic layer and the ionomericmembrane are separately prepared and lastly assembled to form the CCM.

By using the dispersion of the Example 3, the two electrocatalyticlayers of the CCM are separately prepared by the process described inthe Example 4 to prepare the first electrocatalytic layer.

By using the dispersion of the Example 2, the ionomeric membrane isseparately prepared on a support by the process described to prepare theionomeric membrane of the CCM of the Example 4.

The two electrocatalytic layers and the membrane, after the thermaltreatment to evaporate the solvent, are separately heated gradually (10°C./10 min) from the temperature of 65° C. (temperature of solventremoval) to the temperature of 150° C. (annealing temperature). Thetemperature of 150° C. is maintained for 90 minutes.

The CCM is assembled by detaching the membrane from the support andinterposing it between the free surfaces of the two electrocatalyticlayers, which are attached to the respective support. On the freesurface of each support a pressure of 20 bar is applied, heating to 150°C. during the setting in press. The two electrocatalytic layers are inthis way transferred from the respective supports and adhererespectively to each of the two membrane surfaces, forming a CCM.

Determination of the Percentage of the Extractable Ionomer

The preparation of an electrocatalytic layer and of the membrane isrepeated, including the final treatment at 150° C. It is found that thepercentage of the extractable ionomer in both cases is lower than 50%.

Example 5a (Comparative) Performance in Fuel Cell of the CCM of theExample 5 (Comparative)

The CCM of the previous Example is subjected to the above test in fuelcell. The obtained results are the following: at 0.2 A/cm² the voltageis 0.77 V and at 1 A/cm² the voltage is 0.58 V. At the end of the testit is noted that, by disassembling the cell, the structural integrity ofthe CCM is maintained.

Example 5b (Comparative) Integrity Test of the CCM of the Example 5(Comparative) by Prolonged Contact with Water

The CCM of the Example 5 (comparative) is subjected to the above test ofprolonged contact with water. At the end of the test the CCM appearsintegral but bubbles are noticed in correspondence of the interfacesamong the layers. Besides, by leaving the CCM in water at roomtemperature and stirring with a glass rod for two minutes, it isobserved the detachment of the electrocatalytic layers of the membrane.

Example 5c Size Variations of the CCM of the Example 5 (Comparative)

The CCM of the Example 5 (comparative) is subjected to the above test todetermine the size variations. At the end of the test the sizevariations in the two orthogonal directions are the same and equal to8%.

Example 6 (Comparative) Preparation of a CCM According to patentapplication US 2003/0219532

Two membranes are separately prepared on as many supports by using thedispersion of the Example 2 with the process described in the Example 4for preparing the ionomeric membrane, but using the stratifying Braive®knife set at knife height of 550 μm. This heigth is the half of that setin the case of the obtainment of the ionomeric membrane of the Example4. Besides, differently from the Example 4, in this case on the supporton which each ionomeric membrane is prepared by casting there is not thefirst electrocatalytic layer.

Then, on each of the two membranes an electrocatalytic layer is preparedby using the dispersion of the Example 3, by the process described inthe Example 4 for preparing the second electrocatalytic layer on thefree surface of the membrane, using a Kapton® mask having in the middlea square blank space of 10 cm² area.

After having removed the solvent from each electrocatalytic layer, it isfound that it is not possible to remove each membrane with thecorresponding electrocatalytic Layer without compromising the integritythereof.

Therefore the membrane and the respective electrocatalytic layer aretreated as described in the process according to the present invention.It is gradually heated (10°C./10 min) from the temperature of 65° C.(temperature of solvent removal) to the temperature of 150° C. Thetemperature of 150° C. is then maintained for 90 minutes. After thistreatment it is possible to detach the membranes with the respectiveelectrocatalytic layers from the supports.

By a brush an aliquot of the dispersion of the Example 2 is applied tothe free surface of each of the two membranes, said surfaces are thenput into contact and transferred into a press, where they are subjectedto a pressure of 20 bar at a temperature of 80° C. for one hour.

Example 6a (Comparative) Performance in Fuel Cell of the CCM of theExample 6 (Comparative)

The CCM of the previous Example is subjected to the above test in fuelcell. The obtained results are the following: at 0.2 A/cm² the voltageis 0.81 V and at 1 A/cm² the voltage is 0.66 V. At the end of the testit is noted that, by disassembling the cell, the structural integrity ofthe CCM is lost as the membrane with each of the two electrocatalyticlayers divides into the corresponding membranes, containing each of thetwo electrocatalytic layers, which were previously assembled to form theCCM.

Example 6b (Comparative) Integrity Test of the CCM of the Example 6(Comparative) by Prolonged Contact with Water

The CCM of the Example 6 (comparative) is subjected to the above test ofprolonged contact with water. At the end of the test the structuralintegrity of the CCM results lost as the same inconvenience, found bydisassembling the cell at the end of the test described in the Example6a—performance in fuel cell—arises.

Example 7 (Comparative) Preparation of a CCM by Following the Method ofthe Invention Until Obtaining the First Electrocatalytic Layer+IonomericMembrane but by Treating at 150° C. for 90 Minutes before theApplication of the Second Electrocatalytic Layer

The process of the Example 4 is followed until obtaining the assemblyfirst electrocatalytic layer+membrane.

After the solvent removal (65° C. for 30 minutes) the temperature isgradually increased (10° C./10 min) up to 150° C. (annealingtemperature) and this temperature is maintained for 90 minutes.

It is cooled at room temperture and the dispersion of the Example 3 isdeposited on the free membrane surface, by using the process describedin the Example 4 to prepare the second electrocatalytic layer.

It is noted that the dispersion does not homogeneously film andtherefore it is not possible to prepare the second electrocatalyticlayer.

Determination of the Percentage of the Extractable Ionomer on theAssembly First Electrocatalytic Layer+Ionomeric Membrane

The above process is repeated, including the final thermal treatment at150° C. It is found that the percentage of the extractable ionomer islower than 50%.

This Example shows that, in the process of the present invention, thethermal treatment step 4) must be carried out at the end of theformation of the layers forming the CCM.

Example 8 (Comparative)

A part of the polymer in the SO₂F form of the Example 1 is dried at 150°C. for 40 h and subjected to extrusion at 245° C. by Braebender extruderto obtain granules. Then the granules are extruded at 250° C., obtaininga film having a thickness of 30 μm.

A part of the obtained film is treated at 30° C. for 24 h with KOH at10% by weight, then washed with demineralized water and then treated atroom temperature for 24 h with HCl at 20% by weight. At the end it iswashed with demineralized water. In this way the sulphonyl groups of thefilm are converted into acid sulphonic groups.

The process for preparing the electrocatalytic layers and the final CCMassembling of the Example 5 (comparative) is repeated, but by using themembrane obtained by extrusion.

Determination of the Percentage of the Extractable Ionomer on theExtruded Membrane

On a piece of the membrane with acid sulphonic groups obtained from theextruded film the extractable ionomer percentage is determined. It isfound that the percentage of the extractable ionomer is lower than 10%.

Example 8b (Comparative) Integrity Test of the CCM of the Example 8(Comparative) by Prolonged Contact with Water

The CCM of the Example 8 (comparative) is subjected to the above test ofprolonged contact with water. At the end of the test it is found thatthe electrocatalytic layers have detached from the membrane.

Example 8c (Comparative) Size Variations of the CCM of the Example 8(Comparative)

The CCM of the Example 8 (comparative) is subjected to the above test todetermine the size variations. At the end of the test the sizevariations in the two orthogonal directions are respectively 9% in theextrusion direction (MD) and 20% in the orthogonal direction to theextrusion one (TD).

1. A CCM comprising a membrane having on one of the two surfaces a firstelectrocatalytic layer containing a catalyst and on the other one asecond electrocatalytic layer containing a catalyst, wherein: theorthogonal projection of the perimeter of the first electrocatalyticlayer on the second layer is distant from the perimeter of the secondelectrocatalytic layer at least 1 mm, preferably 2 mm in each point ofsaid projection; the electrocatalytic layers and the membrane containing(per)fluorinated ionomers, the CCM having the following features: sizevariations, in the orthogonal directions of the plane xy, lower than15%, more preferably lower than 10% after CCM dipping in demineralizedwater at 100° C. for 30 minutes, dried at 105° C. under vacuum for onehour; the CCM remains substantially unchanged after 60 cycles by dippingin water at 80° C. for 8 hours and then in water at the temperature of25° C. for 16 hours.
 2. A CCM according to claim 1, wherein the(per)fluorinated ionomers are sulphonic ionomers having an equivalentweight in the range from 380 to 1,700 g/eq, preferably from 500 to 1,200g/eq.
 3. A CCM according to claim 1, wherein the (per)fluorinatedionomers contain the following units: copolymers comprising: (A)monomeric units deriving from one or more fluorinated monomerscontaining at least one ethylenic unsaturation; (B) fluorinatedmonomeric units containing sulphonyl —SO₂F groups in amounts such thatthe ionomer has the equivalent weight in the above range; homopolymersformed of monomeric units (B); subsequent ionomer hydrolysis to convertthe —SO₂F groups into sulphonic groups in acid —SO₃H or salified form.4. A CCM according to claim 3, wherein the fluorinated monomers (A) areselected from the following: vinylidene fluoride (VDF); C₂-C₈perfluoroolefins, preferably tetrafluoroethylene (TFE); C₂-C₈ chloro-and/or bromo- and/or iodo-fluoroolefins, preferablychlorotrifluoroethylene (CTFE) and bromotrifluoroethylene; CF₂═CFOR_(f1)(per)fluoroalkylvinylethers (PAVE), wherein R_(f1) is a C₁-C₆(per)fluoroalkyl, preferably selected from trifluoromethyl,bromodifluoromethyl and pentafluoropropyl; CF₂═CFOXperfluoro-oxyalkylvinylethers, wherein X is a C₁-C₁₂ perfluorooxyalkylhaving one or more ether groups, preferably perfluoro-2-propoxypropyl.5. A CCM according to claim 3, wherein the fluorinated monomers (B) areselected from one or more of the following: F₂C═CF—O—CF₂—CF₂—SO₂F;F₂C═CF—O—[CF₂—CX_(A)F—O]_(nA)—(CF₂)_(nB)—SO₂F wherein X_(A)═Cl, F orCF₃; nA=1-10, nB=2, 3; F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F; F₂C═CF—Ar—SO₂F whereinAr is an aryl ring.
 6. A CCM according to claim 3, wherein the sulphonic(per)fluorinated ionomers contain from 0.01% to 2% by moles of monomericunits deriving from a bis-olefin of formula:R₁R₂C═CH—(CF₂)_(m)—CH═CR₅R₆   (I) wherein: m=2-10, preferably 4-8; R₁,R₂, R₅, R₆, equal to or different from each other, are H or C₁-C₅ alkylgroups.
 7. A CCM according to claim 3, wherein the (per)fluorinatedionomers comprise, optionally in admixture with the monomers with —SO₂Fgroups, monomers containing precursor groups which by hydrolysis aretransformed into —COOH groups, selected from the following:F₂C═CF—O—CF₂—CF₂—Y; F₂C═CF—O—[CF₂—CX_(A)F—O]_(nA)—(CF₂)_(nB)—Y whereinX_(A)═C1, F or CF₃; nA=1-10, nB=2, 3; F₂C═CF—O—CF₂—CF₂—CF₂—Y;F₂C═CF—Ar—Y wherein Ar is an aryl group; wherein Y is a precursor groupof the carboxylic group, selected from the following: CN, COF, COOH,COOR_(B), COO⁻Me⁺, CONR_(2B)R_(3B), wherein R_(B) is C₁-C₁₀, preferablyC₁-C₃ alkyl and R_(2B) and R_(3B), equal or different, are H or have themeaning of R_(B), Me is an alkaline metal.
 8. A CCM according to claim3, wherein the membrane and the electrocatalytic layers contain(per)fluorinated ionomers obtainable from ionomers comprising: monomericunits deriving from TFE; monomeric units deriving fromCF₂═CF—O—CF₂CF₂SO₂F.
 9. A CCM according to claim 1, wherein theelectrocatalytic layers comprise an ionomer and a catalyst selectedbetween Pt or a mixture of Pt with one or more metals, as, for example,Ru, Rh, Mo, preferably supported on carbon.
 10. A CCM according to claim1, wherein the ratio by weight between catalyst and ionomer in each ofthe two electrocatalytic layers ranges from 0.5 to 4, preferably from0.5 to 2.5.
 11. A CCM according to claim 9, wherein the ratio mg ofcatalyst metal/cm² of electrocatalytic layer ranges from 0.01 to
 2. 12.A CCM according to claim 11, wherein the ratio (weight in mg of catalystmetal)/(cm² of electrocatalytic layer), when hydrogen is used as fuel,ranges from 0.01 to 0.7 mg/cm², when methanol is used it ranges from 0.3to 1 mg/cm² at the anode side and from 0.5 to 2 mg/cm² at the cathodeside.
 13. A process for preparing CCM of claim 1, comprising thefollowing steps: 1) formation of the first electrocatalytic layer on thesurface of an inert support with the following steps: 1a) preparation ofa first liquid dispersion comprising an ionomer, in acid or salifiedform, and the catalyst, wherein the ionomer concentration (in % by w.),ranges from 0.5% to 40%, preferably from 0.5% to 25%, the ratio byweight between catalyst and ionomer being from 0.5 to 4; 1b) thedispersion prepared in 1a) is placed on the surface of an inert support;1c) removal of the solvent for at least 80% by weight with respect tothe initial solvent of the dispersion, obtaining a firstelectrocatalytic layer having: an extractable ionomer percentage ≧80% byweight, even up to 100% with respect to the total ionomer present in thelayer; a catalyst amount in mg/cm² of electrocatalytic layer between0.01 and 2 mg/cm²; 2) on the first electrocatalytic layer formation ofthe ionomeric membrane through the following steps: 2a) preparation of aliquid dispersion of an ionomer in acid or salified form, equal to ordifferent from that used in step 1a) and having a concentration (% byw.) on the total dispersion weight, from 3% to 40%, preferably from 7%to 25%; 2b) the liquid dispersion 2a) is placed on the free surface ofthe first electrocatalytic layer; 2c) solvent removal for at least 80%by weight as in 1c), obtaining an ionomeric membrane on the top of thefirst electrocatalytic layer; the ionomer percentage extractable afterthis step being ≧80% by weight, even up to 100% with respect to thetotal ionomer present in the first catalytic layer and the membrane; 3)formation of the second electrocatalytic layer on the surface of theionomeric membrane free from catalyst to obtain the raw CCM on the inertsupport through the following steps: 3a) preparation of a second liquiddispersion comprising an ionomer, in acid or salified form, and acatalyst, wherein the ionomer concentration is (% by w.) between 0.5%and 40%, preferably between 0.5% and 25%, the catalyst/ionomer ratio (byw.) being between 0.5 and 4; in said second dispersion the ionomerand/or the catalyst being equal to or different from those used in 1a)and in 2a); 3b) deposition of the liquid dispersion 3a) on the surfaceof the ionomeric membrane free from catalyst prepared in 2) to form thesecond electrocatalytic layer, positioned on the membrane so that theorthogonal projection of its perimeter on the first electrocatalyticlayer is distant from the perimeter of the latter at least onemillimeter, preferably two millimeters in each point of the projection;3c) solvent removal from the second dispersion for at least 80% byweight, as in 1c), until obtaining the second electrocatalytic layer,having: a catalyst amount as in 1c); the ionomer percentage extractableafter this step from the raw CCM being ≧80% by weight, even up to 100%with respect to the total weight of the ionomer present in the CCM; 4)annealing of the raw CCM supported on the inert support at temperaturesfrom 120° C. to 280° C. and obtainment of the CCM by separation of theinert support from the CCM.
 14. A process according to claim 13, whereinin steps 1c) and 2c), the residual amount of solvent is such to obtain anon consolidated system. This means that the system, placed inwater/ethanol (60/40% by weight) at 50° C. for 22 hours, dissolvesalmost completely.
 15. A process according to claim 14, wherein, insteps 1c) and 2c) the residual amount of solvent is up to 5% by weightwith respect to the starting solvent weight.
 16. A process according toclaim 13, wherein the amount of solvent after step 3c) is as in steps1c) and 2c) or in step 3c) the solvent is completely removed.
 17. Aprocess according to claim 13, wherein, by using as ionomers copolymerscontaining units deriving from the sulphonic monomerCF₂═CF—O—(CF₂)₂SO₂F, the annealing of step 4) ranges from 130° C. to280° C., preferably from 140° C. to 180° C., still more preferably from140° C. to 160° C.
 18. A process according to claim 13, wherein theinert preferably non porous support and with smooth surfaces shows thefollowing properties: a size variation in each of the two directions ofthe plane xy, measured at the temperature of step 4) for 15 minutes, nothigher than 2%; it maintains substantially unchanged the mechanicalproperties at the temperatures of step 4); it allows an easy detachmentof the CCM.
 19. A process according to claim 13, wherein in steps 1a),2a) and 3a) the solvent is selected from C₁-C₃, preferably C₃, alcohol,n-propanol and/or iso-propanol, or mixtures, preferably with water, ofC₁-C₃ alcohols.
 20. A process according to claim 19, wherein optionallyother organic solvents can be used provided that they are miscible withwater and/or with C₁-C₃ alcohols.
 21. A process according to claim 13,wherein the dispersion, besides the ionomer and the catalyst, containsother components as (per)fluoropolymer fillers, inorganic fillers,surfactants.
 22. A process according to claim 13, wherein, in step 1b),FIGS. 2 and 2 b, the first electrocatalytic layer is deposited on asupport 3 containing an impression corresponding to the form of theelectrodic layer 4, removing the excess of the dispersion 1a); theheight of the first electrodic layer, after solvent evaporation, beingequal to the height 5 of the impression.
 23. A process according toclaim 13, wherein the formation of the first electrocatalytic layer,FIG. 3, is obtained through the following steps: on the inert support 2is placed a thin and flat sheet 6, wherein a window having the perimeterof a regular flat figure is cut out corresponding to the form of theelectrodic layer; deposit of the dispersion prepared in 1a) in thewindow of the thin sheet, removing the excess of the dispersion by ablade running in contact with the surface of the sheet 6; solventremoval; removal of the sheet
 6. 24. A process according to claim 13,wherein, in order to form the second electrocatalytic layer in step 3,FIG. 3, the sheet 6 placed on the uncoated surface of the membrane isused, wherein the window cut out from sheet 6 has such shape and sizesand is positioned on the membrane so that the orthogonal projection ofits perimeter on the first electrocatalytic layer is distant from theperimeter of the latter at least one millimeter, preferably twomillimeters in each point of the projection.
 25. A process according toclaim 13, wherein in steps 1c), 2c), the solvent removal is carried outby thermal treatment at 45°-95° C., by preferably operating atatmospheric pressure.
 26. A process according to claim 13, wherein afterstep 3c) one passes to step 4) by using a thermal gradient of about 10°C. in the time of 3-30 minutes, preferably 10-20 minutes, preferably 1°C./minute.
 27. Electrochemical cell comprising the CCM of claim
 1. 28.Use of the CCM of claim 1 in electrochemical cells.
 29. Use according toclaim 28, wherein the electrochemical cells are fuel cells.